1. Field of the Invention
The present invention relates to an optical recording/pickup head compatible with optical disks using respective light beams of wavelengths different from each other which records and reproduces information, and more particularly, to an optical recording/pickup head compatible with a recordable compact disk (CD-R) and a digital versatile disk (DVD).
In an optical disk apparatus using a recording medium for storing a large quantity of information, a compact disk (CD) and a digital versatile disk (DVD) have been widely used. Recently, a recordable compact disk (CD-R) and a digital versatile diskxe2x80x94random access memory (DVD-RAM) have been known. As is well known, when recording and reproducing information, laser light having a wavelength of 780 nm is used for a CD-R, and laser light having a wavelength of 650 nm or 635 nm is used for a DVD-RAM. Thus, an optical recording/pickup head compatible with a CD-R and a DVD includes at least two light sources which emit laser light having respectively different wavelengths.
FIG. 1 shows an optical system of a conventional optical recording/pickup head compatible with a CD-R and a DVD. The optical recording/pickup head includes a light source 1 for emitting a first light beam having a wavelength of 635 nm for the recording and reproduction of information with respect to a digital versatile disk (DVD) 8, a light source 11 for emitting a second light beam having a wavelength of 780 nm for the recording and reproduction of information with respect to a recordable compact disk (CD-R) 9, and an objective lens 7 for respectively focusing the first and second light beams emitted from the First and second light sources 1 and 11 on the information recording surfaces of the DVD 8 and the CD-R 9, respectively. A collimating lens 2 collimates the first light beam emitted from the first light source 1 into a parallel light beam and transmits the collimated light to a beam splitter 3. The beam splitter 3 reflects the first light beam incident from the collimating lens 2 to an interference filter prism 4, and the interference filter prism 4 transmits the first light beam, which is a parallel light beam incident from the beam splitter 3, to a quarter-wave plate 5. In more detail, the interference filter prism 4 totally transmits or reflects the incident light beam according to a wavelength of the incident light beam, in which a first light beam of a 635 nm wavelength for a DVD is totally transmitted and a second light beam of a 780 nm wavelength incident from a convergent lens 14 is totally reflected. A thin-film type variable aperture 6 transmits the first light beam, which is the parallel light beam incident from the quarter-wave plate 5, to the objective lens 7. The objective lens 7 focuses the first light beam of the 635 nm wavelength incident in the parallel beam form on the information recording surface of the DVD 8 having a thickness of 0.6 mm. As a result, the first light beam focused on and reflected from the information recording surface of the DVD 8 by the objective lens 7 contains information recorded on the focused position. The reflected first light beam from the information recording surface of the DVD 8 passes through the objective lens 7, the variable aperture 6 and the quarter-wave plate 5, in sequence, and then is incident to the interference filter prism 4. The interference filter prism 4 transmits the first light beam incident from the quarter-wave plate 5 to the beam splitter 3, and the beam splitter 3 makes the first light beam incident from the interference filter prism 4 proceed to a photodetector 10. The photodetector 10 receives the first light beam from the beam splitter 3 and detects information from the first light beam.
The second light beam of the 780 nm wavelength which is emitted from the light source 11 passes through a collimating lens 12 and a beam splitter 13, and then is incident to the convergent lens 14. The convergent lens 14 converges the second light beam incident from the beam splitter 13 to transmit the second light beam to the interference filter prism 4 in the form of a converged beam. The interference filter prism 4 transmits the second light beam incident from the convergent lens 14 to the quarter-wave plate 5 in the form of a divergent beam. The quarter-wave plate 5 transmits the second light beam incident from the interference filter prism 4 to the variable aperture 6. The variable aperture transmits only part of the second light beam of the 780 nm wavelength, and transfers the transmitted second light beam to the objective lens 7 in the form of a divergent beam. The reason why the second light beam is incident to the objective lens 7 in the divergent form, is to focus the second light beam on the information recording surface of the CD-R 9 without having any optical aberration. The variable aperture 6 used in the optical recording/pickup head shown in FIG. 1 will be described in detail with reference to FIG. 2.
FIG. 2 shows in detail the thin-film type variable aperture 6 shown in FIG. 1. The thin-film type variable aperture 6, as shown in FIG. 2, has a structure which can selectively transmit the light beams incident to the regions whose numerical aperture (NA) is less than or equal to 0.6. Among two regions, region 1 is the region whose numerical aperture (NA) is less than or equal to 0.45 and which totally transmits the incident light beam of 780 nm wavelength and 635 nm wavelength. Region 2 is a region having a numerical aperture (NA) of more than 0.45, in which a dielectric thin film is coated. The region 2 is made of a multi-layered thin-film having a thickness of micrometer (xcexcm) order, in which the light beam having the 635 nm wavelength is totally transmitted and that having the 780 nm wavelength is totally reflected. The region 1 is comprised of a quartz (SiO2) thin film in order to remove any optical aberration generated by the dielectric thin film coated region 2. The variable aperture 6 having such a transmission characteristic totally transmits the 780 nm wavelength light being incident there to the region 1 having the 0.45 NA or below and then passes the transmitted light to the objective lens 7, and totally reflects the 780 nm wavelength light incident to the region 2 having the 0.45 NA or above.
The objective lens 7 focuses the second light beam incident from the variable aperture 6 on the information recording surface of the CD-R 9 having a thickness of 1.2 mm, and then forms a beam spot including a main lobe and a side lobe on the information recording surface of the CD-R 9. The reflected second light beam from the information recording surface of the CD-R 9 is transmitted to, the objective lens 7, the variable aperture 6 and the quarter-wave plate 5, in sequence, and then goes to the interference filter prism 4. The interference filter prism 4 reflects the second light beam incident from the quarter-wave plate 5 to the convergent lens 14, and the convergent lens 14 makes the converged second light beam incident to the beam splitter 13. The beam splitter 13 makes the second light beam incident from the convergent lens 14 proceed to a photodetector 15. The photodetector 15 receives the second light beam from the beam splitter 13 and detects information therefrom. Accordingly, the optical system of an optical head of FIG. 1 is capable of performing recording and reproduction with regard to both DVD 8 and CD-R 9
However, the optical recording/pickup head of FIG. 1 forms a beam spot having a side lobe on the information recording surface of the CD-R 9 to thereby generate a crosstalk. Furthermore, the variable aperture 6 is formed so that the light passing through the region 1 and that passing through the region 2 have the same phase difference with each other, in which the region 1 is coated by a quartz thin film and the region 2 is coated by multi-layered dielectric thin film having a thickness of the micrometer order. However, such a fabricating process becomes complicated, and thus causes an increase in production cost.
In order to solve the problems, an object of the present invention is to provide an optical recording/pickup head, including a holographic variable aperture having a first region for totally transmitting incident light beams having respective different wavelengths to an objective lens, and a second region for totally transmitting a light beam having a relatively shorter wavelength, transmitting part of a light beam having the longer wavelength to the objective lens and diffracting the remaining part of a light beam of the longer wavelength, and an objective lens for focusing light beams incident from the variable aperture on the information recording surface of a corresponding optical disk so that a beam spot having a main lobe is formed on the information recording surface of a recordable compact disk (CD-R) and a digital versatile disk (DVD).
Additional objects and advantages of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
To accomplish the above and other objects of the present invention, there is provided n optical recording/pickup head for use with at least two types of optical disks, in which distances from the optical recording/pickup head to the information recording surfaces differ from each other, and information is recorded and reproduced by light beams having respectively different wavelengths, the optical recording/pickup head including a first light source; a first photodetector; a second light source; a second photodetector; an objective lens to focus the first light beam and the second light beam and forms a beam spot on the respective information recording surfaces of two optical disks; optical path alteration means to transfer the first light beam incident from the first light source to the objective lens and to transfer the first light beam incident from the objective lens to the first photodetector, and to transfer the second light beam incident from the second light source to the objective lens and to transfer the second light beam incident from the objective lens to the second photodetector, respectively; and a variable aperture, located on an optical path between the optical path alteration means and the objective lens, to totally transmit the first light beam incident from the optical path alteration means to the objective lens, and to transmit part of the second light beam incident from the optical path alteration means to the objective lens, wherein the objective lens focuses the first and second light beams incident from the optical path alteration means on the information recording surface of a corresponding optical disk so that the beam spot having a main lobe is formed on the respective information recording surfaces of the two types of optical disks, respectively.